HER2 (also known as c-erbB2, ErbB2 or Neu) is a type I transmembrane protein belonging to the family of epidermal growth factor receptors or EGFR, also known as HER1 or ErbB1. Two additional members, HER3 and HER4 complete this family. When HER1, 3 or 4 binds to an EGF-type ligand, its extracellular domain adopts the conformation referred to as “open”, which allows the formation of homodimers and heterodimers. Even if it does not bind to any ligand, HER2 also interacts with other HER receptors linked to a ligand, due to the fact that its extracellular domain is constitutively in “open” conformation.
The dimerisation directed by the extracellular domain leads to interaction of the intracellular kinase of HER receptors and to the subsequent transphosphorylation of some tyrosine residues. These phosphotyrosines, act as couplers of a group of intracellular phosphotyrosine-binding proteins. The interactions established at the plasmatic membrane are transduced to the cell nucleus by means of different signalling routes, such as the protein kinase route activated by mitogen activated protein kinase (MAPK), the protein kinase route activated by stress (JNK), phospholipase C gamma, etc. All of these signalling circuits control the expression of genes that act in a coordinated way to modify determining aspects of the state, of the cell, such as cell proliferation, migration, survival and adhesion. Thus, depending on the cell context, activation of the HER receptors results in a dramatic cell response, which can range from transformation into a malignant cell to a premature senescence.
In addition to the canonical signalling mode, the HER receptors or fragments thereof can be endocyted and transported to the nucleus, where they can directly regulate the expression of certain genes. This fact has been observed for the specific case of HER2 (Wang et al., “Binding at and transactivation of the COX-2 promoter by nuclear tyrosine kinase receptor ErbB-2”, Cancer Cell—2004, Vol. 6, pp. 251-261), as well as for a carboxy terminal fragment (CTF), which consists of a HER4 receptor truncated form, which includes the entire cytoplasmic part (Linggi et al., “ErbB-4 s80 intracellular domain abrogates ETO2-dependent transcriptional repression”, J. Biological Chemistry—2006, Vol. 281, pp. 25373-25380).
In human breast tumours, a series of carboxy terminal fragments or CTFs has been often found, which are assumed to include the transmembrane and cytoplasmic domains of HER2. (Molina et al, “NH(2)-terminal truncated HER2 protein but not full-length receptor is associated with nodal metastasis in human breast cancer”, Clinical Cancer Research—2002, Vol. 8, pp. 347-353). It is also known that patients with breast cancer expressing the HER2 CTFs, or what comes to be the same truncated forms which do not include the N-terminal end of HER2 (HER2 CTFs), have a greater probability of developing metastasis (Molina et al. 2002—supra) and a worse prognosis than those patients who mainly express the complete form of HER2 (Saez et al., “p95HER2 predicts worse outcome in patients with HER2-positive breast cancer”, Clinical Cancer Research—2006, Vol. 12, pp. 424-431).
Therefore, it is very important to be able to detect the presence of HER2 in tumours on time and even more to determine whether it is in a complete or truncated (CTF) form.
Nowadays, at the level of routine clinical tests antibodies are used to detect the presence of the complete form of HER2, in order to determine the type of breast tumour in question. In the event of detecting the presence of HER2, the recommended therapy consists of administering therapeutic monoclonal antibodies, such as Genentech's trastuzumab. The use of this monoclonal antibody for the treatment of cancer is described in the application WO 8906692 on behalf of Genentech. WO 8906692 describes monoclonal or polyclonal antibodies directed against the extracellular region of HER2, this extracellular region matching with the complete external part of the protein, which is its N-terminal end. As previously mentioned, the epitopes recognised by the antibodies cited in WO 8906692 are antigenic regions of the extracellular domain of the HER2 complete form and are not included in truncated forms or carboxy terminal fragments of HER2. Therefore, with the antibodies and the diagnosis method described in WO 8906692 it will not be possible to detect the presence of truncated HER2 (CTFs). Also, in breast tumours where said CTFs of HER2 are expressed, antibodies such as trastuzumab are not therapeutic, since they do not recognise any epitope. This fact explains the resistance to the treatment with trastuzumab observed in patients expressing truncated forms (CTFs) of HER2.
Patients who express truncated forms or CTFs of HER2 should be treated with alternative therapies in order to prevent the poor prognosis numbers observed by Saez et al 2006 (supra). For the purpose of detecting (diagnosing) and treating as soon as possible people having tumours where HER2 truncated forms are expressed, it is very interesting to be able to distinguish what form of HER2 is expressed in order to act in consequence.
The document of Anido et al., “Biosynthesis of tumorigenic HER2 C-terminal fragments by alternative initiation of translation”, European Molecular Biology Organization (EMBO) Journal—2006, Vol. 25, pp. 3234-3244, describes that in addition to the fragment generated by the action of the alpha-secretases on the HER2, which gives rise to a truncated form (CTF) known as P95 that includes the transmembrane and cytoplasmic fragment of the receptor, two truncated forms (CTF) are also generated through a mechanism of alternative initiation of translation starting from two methionines located upstream and downstream, respectively, of the transmembrane domain of HER2. Specifically, the methionines for the alternative initiation of translation correspond to methionine 611 and methionine 687 of the amino acid sequence with access number M11730.1 of the UniGene database of the National Center for Biotechnology Information (NCBI). This document also shows that these alternative forms of the HER2 receptor (CTFs) are present in breast tumours. In particular, it indicates that the most abundant corresponds to the form known as CTF687, or in other words, to the protein obtained by the alternative initiation of translation starting from the methionine in position 687. Anido et al. proposes as therapy the use of inhibitors of the tyrosin kinase activity of HER2, such as lapatinib, in order to minimise the growth of the tumours expressing these truncated forms of HER2. The inhibitors of tyrosin kinases act by interacting with the C-terminal end of the HER2 receptor that is present in an integral manner in both the complete receptor and in the CTFs derived from it or produced by alternative initiation of the translation.
The document of Scaltriti et al, “Expression of p95HER2, a truncated form of the HER2 Receptor, and response to anti-HER2 therapies in breast cancer”, Journal of National Cancer Institute—2007, Vol. 99, pp. 628-368, represents an example of a study into an alternative methodology for detecting the presence of one of the HER2 receptor truncated forms and lists some of the possible causes of resistance to the treatment with trastuzumab (Herceptin). In particular, it emphasises the accumulation of the p95HER2 fragment (product of the proteolysis of HER2 by alpha-secretases) and other truncated forms of the receptor, which do not have the extracellular domain recognised by Trastuzumab. Scaltriti proposes a method of immunofluorescence to detect the p95HER2 fragment (product of proteolysis by alpha-secretases). This new detection method can be carried out on sections of tissue embedded in paraffin and fixed with formalin following clinical protocols. The new methodology arises from observing that the truncated form p95HER2, and not the entire form of the receptor, is located in both the plasmatic membrane and the cytoplasm of the cell. This method proposes comparing whether there is expression of p95HER2 by means of staining the detected cytoplasm with an anti-HER2 antibody that binds the receptor's cytoplasmic domain; and confirming these results with those of a detection with an anti-cytokeratin antibody, a protein whose distribution has been extensively used as a tool for the diagnosis of tumours. However, it is not clear whether this method efficiently distinguishes those tumours expressing the complete form of HER2 from those that express the truncated forms.
There is a need to locate new and efficient targets for diagnosis and therapy, properly correlated to the type of cancer in question and that make it possible to treat on time with efficient therapies those tumours having the worst prognosis, discarding from the outset those therapies that have been seen not to be effective.
The present invention offers benefits related to the problems cited above and represents a novel solution in the early classification of cancer, specifically breast cancer.